This invention relates to a system and method for simultaneous prediction of dent resistance and oil canning resistance of automotive roof panels, and in particular, to how the effect of roof bow placement, curvatures of the panel roof, thickness of the roof, and steel grade affect dent and oil canning resistance.
Among the performance problems that can occur in an automobile panel, and in particular, a roof panel, are oil canning, also known as snap-through buckling, and dents. Snap-through buckling is an inherent part of light gauge formed metal products, in particular, those with broad flat areas such as an automobile roof panel. Obviously, snap-through buckling and dents can mar the appearance of a panel, produce unwanted noise, and may reduce consumer satisfaction level.
Snap-through buckling is a complicated instability phenomenon that occurs in relatively flat panels and is experienced by a number of industries which deal with large and shallow panels. Fundamentally, snap-through buckling is caused because of compressive stresses placed upon a circular arch. These compressive stresses may be caused by external loading or by residual stresses resulting from manufacturing. The result of this instability is dependent upon the type of loading, curvature of the panel, compliance with the supporting structure, as well as other variables. The problem with snap-through buckling on shallow arches has been studied previously in some detail. Although useful to explain the phenomenon of snap-through buckling, the boundary and loading conditions used in prior studies are not simulative of the in-service loading conditions experienced in the automotive industry. Accordingly, the results could not be used to evaluate snap-through buckling resistance of automotive panels.
Resistance to denting and snap-through buckling are important properties for closure panels. Dent resistance of automotive closure panels has been studied extensively and is known to be dependent on the steel grade, thickness, and panel curvature. In many cases, the ability of a higher strength steel grade to achieve weight reduction by reducing the thickness is limited by the stiffness of the panel and its resistance to snap-through buckling. Snap-through buckling is a phenomenon that occurs on loading of a panel, when the panel resistance suddenly decreases with increasing imposed deflection. In some instances, the drop-in load is accompanied by the release of a sound.
Historically, dent resistance and snap-through buckling resistance were evaluated by physical testing of panels according to Auto/Steel Partnership guidelines. Physical testing of a number of prototype parts would give the best indication of expected dent resistance and snap-through buckling resistance during service, but it requires significant time and effort. In addition, different types of steel to be prototyped need to be identified and procured from a steel mill for the testing. Stamping and assembly trials also require coordination in the middle of existing production runs, and then, finally, prototype parts could actually be tested. Over the last decade, Finite Element Analysis (FEA) has been used extensively for evaluation of these performance metrics. The analysis methodologies and pass/fail bogeys vary with the manufacturer and also depend on the panel type and vehicle class. Typically, a full vehicle structural model is truncated to obtain the exposed panel structural model. The model is then further refined at the localized areas of loading and submitted for analysis and the results post-processed. Using this typical approach, analysts might take a few weeks to arrive at an appropriate solution to determine a thickness-grade combination for a given exposed panel.
Meeting requirements for snap-through buckling resistance, stiffness and dent resistance are important drivers for most automotive Original Equipment Manufacturers (OEMs) in making material decisions for exposed panels. As discussed, dent resistance has been shown to be dependent on panel curvature, steel grade, thickness and stretch imparted to the panel door during the stamping process. The bake hardening of steel grades has been one method of increasing the panel strength to decrease the weight of outer panels while meeting the dent resistant performance, and increasing the panel strength from the paint bake cycle has been used effectively.
The owner of the present patent application has previously developed a model for prediction of dent resistance for a number of steel grades. The model has been shown to be reasonably accurate in comparison with physical test results; however, before now the model has been applicable only to doors. The system and method is described in U.S. Pat. No. 7,158,922 B2 to Sadagopan et al., which is incorporated herein in its entirety by reference.
Snap-through buckling is characterized by a drop in resistance of the panel in response to an imposed deflection under localized loading conditions. As the thickness of the sheet metal decreases, resistance to snap-through buckling also decreases, and in some cases, the drop in resistance is accompanied by a significant noise. Unlike dent resistance, resistance to snap-through buckling is dependent on the panel geometry, support conditions and thickness. The steel grade is relatively unimportant to snap-through buckling. In many instances, the ability to down gauge a panel is limited by its snap-through buckling resistance.
Accordingly, an object of the present invention is to provide predictive guidelines for snap-through buckling resistance of roof panels. Another object of the present invention is to expand the on-line dent resistance model previously developed for door panels. Utilization of such tools enables optimization and selection of radii of curvature, steel grade, thickness, and design decisions during the styling stage of vehicle development to meet stiffness, snap-through buckling and dent resistance criteria for panels. An advantage of the subject invention is to allow an OEM to avoid spending significant analysis time while minimizing the need for costly fixes, adjustments, and changes later on in program development. A further object of the invention is to provide reasonable results for idealized geometries and loading conditions to analyze possible scenarios in relation to the steel grade, steel thickness, panel styling, and design options that can be performed in a much shorter time frame than conventional analysis techniques will allow.
A further object of the invention is to provide predictive models of snap-through buckling and dent resistance for automotive roof panels where the models can be combined in a single-user interface. Another object of the invention is that the predictive model correlate favorably with FEA when the loading is located in the center of the panel. The predictive model shows that resistance to snap-through buckling of roof panels can be influenced by placement of appropriate roof bows, and that snap-through buckling can be avoided by placing roof bows closer to one another.